The invention relates to improved linear alkylene terephthalate polymers containing a reinforcing filler and more particularly relates to improved glass fiber-filled polybutylene terephthalate compositions. It is especially concerned with glass fiber-filled polybutylene terephthalate compositions exhibiting diminished warpage on molding without a deleterious effect on the other properties of the glass fiber-filled polybutylene terephthalate.
Polybutylene terephthalate containing glass fiber filler as reinforcing filler is widely employed as an engineering thermoplastic composition, for example in the preparation of molded articles on account of its excellent tensile strength, high heat distortion temperature, good processability and excellent chemical resistance. Glass fiber-filled polybutylene terephthalate however suffers from the disadvantage of undergoing a high degree of warpage on molding. This problem which distinguishes glass fiber-filled polybutylene terephthalate from other comparable glass fiber-filled alkylene terephthalate polymer e.g. glass fiber-filled polyethylene terephthalate, is believed to be principally due to the effect of the exceptionally high rate of polymer crystallization during the molding of the polybutylene terephthalate compositions (which crystallization rate is, in contrast, for example, to the well known slow rate of crystallization of comparable polyethylene terephthalate compositions). The exceptionally high rate of crystallization occurring with the polybutylene terephthalate compositions promotes anisotropy in the molded glass fiber-filled polybutylene terephthalate which in turn, gives rise to uneven shrinkage in the molded product thereby resulting in enhanced product warpage. (The foregoing explanation is offered to clarify the nature of the high product warpage encountered on molding conventional glass fiber-filled polybutylene terephthalate. However, the invention is not bound to any particular theoretical explanation of warpage in glass fiber-filled polybutylene terephthalate).
It is the principal object of this invention to provide a glass fiber-filled polybutylene terephthalate composition exhibiting diminished warpage on molding.
It is another object of this invention to provide a warpage-inhibited glass fiber-filled polybutylene terephthalate which retains substantially the known desirable beneficial properties of the prior art glass fiber-filled polybutylene terephthalate.
These and other objects and advantages will be apparent from the following description of the invention.